From the U.S. Pat. No. 6,124,054, a fuel cell system is known, in which the waste gases emerging from the anode and the cathode region are made to converge and are fed to a catalytic conversion process. The underlying difficulty regarding humidification is only discussed in detail in the U.S. Pat. No. 5,503,944 A1 referenced therein. It describes carrying out the humidification process from an ultrapure-water (cooling) circuit through the cathode flow fields having a porous design. Thus, the gases are supplied in a dry process. However, it is difficult to protect such an ultrapure-water circuit from frost, so that it has considerable disadvantages with regard to its suitability for temperatures below the freezing point. Such humidification processes are certainly not conceivable in mobile applications, such as in vehicles.
A similar type of design is also known from the German Patent Application DE 100 24 570 A1. It provides for additionally directing the waste gases from the catalytic combustion to an expander in order to utilize the residual energy contained in the same. It also describes an anode design where the active surface area of the anode is reduced by at least one step in the direction of fuel flow, so that the fuel is utilized at a higher proportion in the fuel cell. A comparable approach providing this type of cascading anode design is also derived from the German Patent Application DE 197 21 817 A1.
Neither of the two last-mentioned publications allow for the underlying difficulty regarding humidification which, however, is essential for the operation of a PEM fuel cell. Namely, if a gas stream is directed to a PEM fuel cell which has not been humidified, thus which is able to absorb water from the PEM (proton-conducting membrane or polymer electrolyte membrane), it dries out the PEM. It is effectively damaged as a result.
For that reason, the International Patent Application WO 01/11216 A2 describes humidifying the gas streams using membrane humidifiers. However, these devices are very large and costly, in particular when air is used as an oxidant stream, due to the comparatively large volume to be humidified that also includes inert components. This constitutes a significant disadvantage for low cost, compact fuel cell systems, as are required in vehicles, for example, for purposes of energy supply and propulsion.
In addition, from the German Examined Application DE 100 55 253 B4, as well as the non-prepublished German Applications DE 103 46 594 and DE 10 2004 058 117 of the Applicant, supplementary cathode components are known, which make it possible to reduce the outlay required for humidifying the oxidant by providing appropriate flow guidance therefor. These supplementary components of the oxidant distributor structures, denoted as “injection flow field,” allow the oxidant to flow into the cathode in each of the fuel cells at least two different locations. This reduces the volumetric flow at each of the inflow locations, thereby minimizing the risk inherent to the particular inflow region of the oxidant, of the PEM drying out at certain spots. Finally, the components allow water retention in the region of the cathode, thereby reducing the demands placed on the humidification process.